The transfer mold method has been conventionally introduced as a method of molding resin to seal electronic parts. This method is usually performed as described below using a mold for the resin molding according to a first conventional example as shown in FIGS. 1 and 2.
A stationary upper mold 51 and a movable lower mold 52 are preheated to a predetermined resin molding temperature by a heater 64 while the upper and lower molds 51 and 52 are opened as shown in FIG. 1.
Next a lead frame 62 to which an electronic part 63 is attached is fittingly set in a predetermined position in a concave portion 61 provided in a molding surface of the lower mold 52, and a resin tablet 67 is supplied into a pot 53.
Next, the lower mold 52 is moved upwards to close the upper and lower molds 51 and 52 as shown in FIG. 2. At this time, the electronic part 63 and the lead frame around it are fittingly set inside upper and lower mold cavities 55 and 56 oppositely formed in both of the molding surfaces, and the resin tablet 67 inside the pot 53 is heated to gradually melt.
Next, the resin tablet 67 inside the pot 53 is pressurized by a plunger 54. Then, the melted resin material is injected into both cavities 55 and 56 through a resin path 57 provided between the pot 53 and the upper mold cavity 55. Then, the electronic part 63 and the lead frame 62 carrying part 63 are sealed in the resin sealing body molded corresponding to the shape of both cavities 55, 56. Accordingly, after a time required for hardening of the melted resin material has passed, the hardened molded resin body, the lead frame 62, and the hardened resin inside the resin path are respectively separated from the molds by ejector pins 65 simultaneously with opening the upper and lower molds.
Air exists inside a space formed between the molding surfaces when the upper and lower molds 51 and 52 are closed, that is, a space in the molds including the pot 53, the resin path 57 and the upper and lower cavities 55 and 56. Accordingly, it has been a problem in such conventional resin molding that voids or defective portions are formed inside or on the surface of a molded resin body because the remaining air mixes into the melted resin material. Accordingly, said space is configured so that remaining air can be naturally pushed out through an air vent 58 utilizing the effects of transmitting, ejecting and filling the melt resin material inside the pot 53 through resin path 57 into the upper and lower cavities 55 and 56 by communicating the above-mentioned mold space namely the cavity 55 to the outside of the mold and the appropriate air vent 58.
As described above, the air existing in pot 53, resin path 57 and both of the cavities 55 and 56 can be naturally pushed out from air vent 58. Actually, however, especially the formation of inner voids in molded resin bodies and the like cannot be effectively prevented. Accordingly, the problems of damaging the humidity resistance and appearance of products produced as described above have not been completely solved in practice.
In order to avoid formation of voids inside molded resin bodies, the following further improved methods are possibilities. For example, by applying a predetermined resin pressure to a melted resin material injected to fill both cavities 55 and 56, the bubbles which entered into the melted resin material are compressed to a neglectable degree for implementing conditions under which inner voids are not apparently formed. In said further method, however, although the advantage of void removal can be realized to some extent, problems are caused by the fact that apparatus, mechanisms and the like with a high lasting quality are necessary for applying a high resin pressure and furthermore that the work is risky.
The air existing in pot 53, resin path 57 and both cavities 55 and 56 is forced to be exhausted to the atmosphere, to prevent the air from contaminating the melted resin material. Such a method is excellent in that the air does not easily mix into the melted resin material since the air existing in the pot, the resin path and both cavities is exhausted. As compared to the conventional method of naturally exhausting the air to the atmosphere, the formation of inner voids in molded resin bodies can be prevented more effectively by resin pressure. However, despite that the formation of inner voids can be effectively avoided, the forced exhaustion method is not actually applied or practiced as a method of molding resin seals for electronic parts. Even if the above method is applied in practice, the effects as described above cannot be obtained actually.
Also, when the seal molding operation for sealing of electronic parts with resin molding is not surely performed, or when the condition of contact between the molded resin body and the lead frame of the electronic parts is insufficient, the humidity resistance of products is also degraded similarly to the above-described case where voids are formed in molded resin bodies. Furthermore, for example, in the working process of bending an outer lead protruding from a molded resin body, cracks may be produced in the molded resin body due to the bending work force applied to a base portion of the outer .lead, or the portion might be broken. Accordingly, because of disadvantages such as those mentioned above, it is a serious problem that the high quality and high reliability strongly demanded for this kind of products cannot be obtained.
Next, a second conventional example of a method of molding a resin seal of electronic parts according to the transfer mold method and apparatus thereof will be described referring to FIGS. 3 and 4.
The conventional apparatus shown in FIGS. 3 and 4 uses a multi-plunger type system and is described below whereby reference is made to U.S. Pat. No. 4,793,785 (Osada), issued Dec. 27, 1988.
The known apparatus includes a stationary mold 1 and a movable mold 2 disposed below and opposed to the mold 1. These molds 1 and 2 include bases 3 and 4, respectively, which are provided with heating means 5 and 6 such as oil heaters, electric heaters or the like.
The stationary mold base 3 and the movable moldbase 4 are respectively provided with a stationary chase block 7 and a movable chase block 8 movably fitted therein by a mortise joint.
The movable chase block 8 has a plurality of pots 9 each of which is provided with a pair of right and left lower cavities 10 in the vicinity thereof. A heater heats the lower cavity 10. Disposed below the chase block 8 is a lower ejector plate 12 having ejector pins 12a for ejecting resin bodies molded in the lower cavities 10, and a plunger holder 13 supporting thereon plungers 13a for applying a pressure to the resin material supplied to the pots 9. The ejector pins 12a are fittingly inserted in bores 14 extending through the chase block 8 and communicating with the lower cavities 10. Each plunger 13a is inserted through bores 15, 16 formed in the movable mold base 4 and the ejector plate 12, respectively, and fitted in the pot 9. The tenon 8a of the chase block 8 has at least one vertical screw hole not shown. When the tenon 8a is fitted in the mortise 4a to install the chase block 8 in the base 4, the screw hole is in register with a vertical bolt hole 18 formed in the base. Accordingly, the block 8 can be properly fixed in position to the base 4 by screwing a positioning bolt 19 into the screw hole 17 through the bolt hole 18.
The stationary chase block 7 has a pair of right and left upper cavities opposed to each pair of lower cavities 10. A heater 6, is provided in the vicinity of the upper cavities. Disposed above the chase block 7 are an upper ejector plate 22 having ejector pins 22a, a support pin 22b for the plate 22, and a spring 23 for depressing the plate 22 through the pin 22b. The ejector pins are fittingly inserted through vertical bores 25 formed in the block 7 and communicate with the upper cavities 20 or culls, not shown, opposed to the pots 9. The ejector plate 22 is depressed by the force of the spring 23 when the molds are opened, whereby the resin bodies molded in the upper cavities, the culls and gates, not shown, through which the cavities communicate with the culls, are ejected from the cavities.
At this time, the lower ejector plate 12 is pushed up by the ejector bar 12b which is fixed at a portion 12c located on the outer side of the movable mold 2, whereby the molded resin bodies in the lower cavities 10 are ejected. However, when the movable mold 2 is raised into clamping contact with the stationary mold 1 at the plane of their parting line P.L., upper and lower return pins, not shown, opposed to and mounted on the upper and lower ejector plates 22, 12, retract the plates 22, 12 upwardly and downwardly respectively.
The stationary cavity block 7 and the stationary mold base 3 have a tenon 7a and a mortise 3a which are the same as the tenon 8a and the mortise 4a of the movable chase block 8 and the movable mold block 4. A fixing device comprising a screw hole, a bolt hole, and a bolt identical with the above-mentioned device, is provided for fixing the movable chase block 8 to the movable mold base 3.
The movable chase block 8 is fittable into and removable from the base 4, for example, together with the lower ejector plate 12 when the plungers 13a are in a downwardly moved position.
The stationary chase block 7 is fittable into or removable from the base 3, for example, together with the upper ejector plate 22 after the support pin 22b has been removed from the plate 22.
The operation of sealing electronic parts with resin according to the second conventional example is almost the same as that of the above described first conventional example.
Inside and outside of the cavity described above, usually communicate with each other through an appropriate air vent and the air inside the cavity is naturally pushed out through the air vent through the effect of the resin injection and filling, when injecting to fill both of the cavities with the melted resin material.
The electronic part and the lead frame around it are encapsulated inside the resin body molded corresponding to the shape of both cavities. After the resin hardens, the lower mold is moved downward to open the upper and lower molds, and almost simultaneously, the molded resin body and the lead frame inside both cavities and the hardened resin inside the resin path are released by a releasing mechanism, respectively.
As described above with reference to the first conventional example, the remaining air existing in the pot, the resin path and both cavities is naturally exhausted outside through the air vent. Venting the remaining air, however, is a problem because the formation of voids in the molded resin bodies cannot be effectively prevented since the exhausting action is not satisfactory.
In order to solve the problem of the formation of voids in the molded resin bodies, for example, it is a possibility to exhaust the remaining air existing in a pot, a resin path, and both cavities by force using a vacuum source.
However, although exhausting with a vacuum source is excellent in that the above-mentioned remaining air inside the space of the mold structure is not easily mixed into the melt resin material, it is not actually put into practice because of the following technical problems.
That is to say, upper and lower molds of a mold for resin molding are opened and closed every time the resin is molded, and for using the above-stated vacuum source, it is necessary to seal by suitable seals at least the pot, the resin path, and cavity portion from the outside air when clamping both molds and exhaust the remaining air and moisture within the so enclosed space.
As the above-mentioned seals for example, the following are possibilities; O-rings are provided on the molding surfaces of both molds, which are depressed when closing the molds to exclude the outside air from the area defined thereby, or fitting concave and convex portions are oppositely provided on molding surfaces of both molds, which are fit to each other when closing the molds, and these O rings provided on fitting surfaces of the concave and convex portions are depressed to exclude the outside air from the area set thereby.
However, when providing outside air excluding seals such as O-rings and fitting concave and convex portions and so forth on molding surfaces of both molds, closing of both molds and the action of excluding the outside air by the outside air excluding vacuum source are performed almost simultaneously. Accordingly, even when an abnormality occurs, for example when a lead frame positioned on a mold surface is not set at a predetermined position or state due to some reasons, the mold closing action and the outside air excluding action are performed almost simultaneously, resulting in a problem that the molding surfaces of both molds, the lead frame and the outside air excluding seals may be damaged.
Also, if attempting to fix the outside air excluding seals on the molding surfaces of both molds, a problem is caused that the work of attaching and replacing the chase blocks on both of the molds is troublesome. Furthermore, if the chase blocks are often to be replaced due to the production of many kinds and small numbers of electronic parts, there is a problem that the total productivity is reduced because the chase block replacing work takes time and effort.
When supplying a resin tablet into a pot and melting the resin tablet by heating and pressurizing, for the purpose of aiding the thermally melting effect, the resin tablet is pre-heated before supplying the resin tablet into the pot.
Since a melted resin layer is formed on a surface of the pre-heated resin tablet, however, when supplying the same into the pot and applying heat for heating the molds, large amounts of air and moisture included inside the resin tablet cannot be extracted to the outside because it is prevented by the above-mentioned melted resin layer. Accordingly, a large amount of air and moisture mixes into the melted resin material whereby voids are formed on a surface portion and an inside portion of a molded resin body, which is a problem.
When sealing the area of the pot, the resin path and the cavity portion from the outside and exhausting the remaining air and moisture inside that area by a vacuum source when both molds are completely closed, the remaining air and moisture are exhausted to the atmosphere through an air vent formed of fine gaps provided between molding surfaces of both molds, because it is extremely difficult that the air and moisture go out from the contact surface of both molds, since surfaces of both molds are connected closely and strongly because of the demand in the resin molding of preventing occurrence of resin flash on both mold surfaces.
Accordingly, when both molds are completely closed, even if applying the above-described exhausting effect by a vacuum source, the efficiency of exhausting the remaining air and moisture existing between molding surfaces of both molds is inferior, resulting in a problem that a vacuum condition cannot be actually implemented inside the above-stated area.